The opening and closing of heart valves occur primarily as a result of pressure differences. For example, the opening and closing of the mitral valve occurs as a result of the pressure differences between the left atrium and the left ventricle. During ventricular diastole, the venous return of blood from the pulmonary veins into the left atrium causes the pressure in the atrium to exceed that in the left ventricle. As a result, the mitral valve opens and allows blood to enter the left ventricle. As the left ventricle contracts during ventricular systole, the intraventricular pressure rises above the pressure in the atrium and pushes the mitral valve shut.
When the high pressure produced by contraction of the left ventricle pushes the valve leaflets too much, the leaflets become everted and prolapse results. This is normally prevented by contraction of the papillary muscles within the left ventricle, which are connected to the mitral valve leaflets by the chordae tendineae (chords). Contraction of the papillary muscles is simultaneous with the contraction of the left ventricle and serves to keep healthy mitral valve leaflets tightly shut at peak contraction pressures.
Mitral valve malfunction can result from the chords becoming stretched and, in some cases, tearing. When a chord tears, for example, the result is a flailed or prolapsed leaflet. Also, a normally-structured valve may not function properly because of an enlargement of the valve annulus. This condition is referred to as a dilation of the annulus, and generally results from heart muscle failure. In addition, the valve may be defective at birth or because of an acquired disease.